<HTML>
<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A> 
</CENTER>






<HR>

<H3>pair_style body command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style body cutoff 
</PRE>
<P>cutoff = global cutoff for interactions (distance units)
</P>
<P><B>Examples:</B>
</P>
<PRE>pair_style body 3.0
pair_coeff * * 1.0 1.0
pair_coeff 1 1 1.0 1.5 2.5 
</PRE>
<P><B>Description:</B>
</P>
<P>Style <I>body</I> is for use with body particles and calculates pairwise
body/body interactions as well as interactions between body and
point-particles.  See <A HREF = "Section_howto.html#howto_14">Section_howto 14</A>
of the manual and the <A HREF = "body.html">body</A> doc page for more details on
using body particles.
</P>
<P>This pair style is designed for use with the "nparticle" body style,
which is specified as an argument to the "atom-style body" command.
See the <A HREF = "body.html">body</A> doc page for more details about the body
styles LAMMPS supports.  The "nparticle" style treats a body particle
as a rigid body composed of N sub-particles.
</P>
<P>The coordinates of a body particle are its center-of-mass (COM).  If
the COMs of a pair of body particles are within the cutoff (global or
type-specific, as specified above), then all interactions between
pairs of sub-particles in the two body particles are computed.
E.g. if the first body particle has 3 sub-particles, and the second
has 10, then 30 interactions are computed and summed to yield the
total force and torque on each body particle.
</P>
<P>IMPORTANT NOTE: In the example just described, all 30 interactions are
computed even if the distance between a particular pair of
sub-particles is greater than the cutoff.  Likewise, no interaction
between two body particles is computed if the two COMs are further
apart than the cutoff, even if the distance between some pairs of
their sub-particles is within the cutoff.  Thus care should be used in
defining the cutoff distances for body particles, depending on their
shape and size.
</P>
<P>Similar rules apply for a body particle interacting with a point
particle.  The distance between the two particles is calculated using
the COM of the body particle and the position of the point particle.
If the distance is within the cutoff and the body particle has N
sub-particles, then N interactions with the point particle are
computed and summed.  If the distance is not within the cutoff, no
interactions between the body and point particle are computed.
</P>
<P>The interaction between two sub-particles, or a sub-particle and point
particle, or betwee two point particles is computed as a Lennard-Jones
interaction, using the standard formula
</P>
<CENTER><IMG SRC = "Eqs/pair_lj.jpg">
</CENTER>
<P>where Rc is the cutoff.  As explained above, an interaction involving
one or two body sub-particles may be computed even for r > Rc.
</P>
<P>For style <I>body</I>, the following coefficients must be defined for each
pair of atoms types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in
the examples above, or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands:
</P>
<UL><LI>epsilon (energy units)
<LI>sigma (distance units)
<LI>cutoff (distance units) 
</UL>
<P>The last coefficient is optional.  If not specified, the global cutoff
is used.
</P>
<HR>

<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
</P>
<P>For atom type pairs I,J and I != J, the epsilon and sigma coefficients
and cutoff distance for all of this pair style can be mixed.  The
default mix value is <I>geometric</I>.  See the "pair_modify" command for
details.
</P>
<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
shift, table, and tail options.
</P>
<P>This pair style does not write its information to <A HREF = "restart.html">binary restart
files</A>.
</P>
<P>This pair style can only be used via the <I>pair</I> keyword of the
<A HREF = "run_style.html">run_style respa</A> command.  It does not support the
<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
</P>
<HR>

<P><B>Restrictions:</B>
</P>
<P>This style is part of the BODY package.  It is only enabled if LAMMPS
was built with that package.  See the <A HREF = "Section_start.html#2_3">Making
LAMMPS</A> section for more info.
</P>
<P>Defining particles to be bodies so they participate in body/body or
body/particle interactions requires the use of the <A HREF = "atom_style.html">atom_style
body</A> command.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>, <A HREF = "fix_rigid.html">fix_rigid</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
